CO(CONSTANS) gene acts between the circadian clock and genes controlling meristem identity.In Arabidopsis, CO transcription factors defined by two conserved domains. The first is a zinc finger region near the amino terminus that resembles B-boxes, which regulate protein-protein interactions in several animal transcription factors. The second is a region of 43 amino acids near the carboxy terminus termed the CCT (CO, CO-like, TOC1) domain. Studies using green fluorescent protein fusions show that the CCT domain is involved in nuclear localization of the CO protein but must have an additional role because the late-flowering co-7 mutant, which has an altered CCT domain, correctly localizes the protein.